The backfire quadrifilar antenna is well-known and has particular application in the transmission and reception of circularly polarised signals to or from orbiting satellites. British Patent Application No. 2292638A discloses a miniature quadrifilar antenna having four half-wavelength helical antenna elements in the form of narrow conductive strips plated on the surface of a cylindrical ceramic core. Connecting radial elements on a distal end face of the core connect the helical elements to a coaxial feeder passing axially through the core in a narrow passage. The helical elements are arranged in pairs, the elements of one pair having a greater electrical length than those of the other pair by virtue of their following a meandering course, all four elements being connected to the rim of a conductive balun sleeve which rim describes a circle lying in a plane perpendicular to the antenna axis. British Patent Application No. 2310543A discloses an alternative antenna in which the balun sleeve has a non-planar rim, the helical elements being simple helices which terminate in peaks and troughs respectively of the rim in order to yield elements of the required different lengths.
The fact that the pairs of elements have different electrical lengths results in a phase difference between the currents in the respective pairs at the operating frequency of the antenna, and it is this phase difference which makes the antenna sensitive to circularly polarised radiation with a cardioid radiation pattern such that the antenna is suited to receiving circularly polarised signals from sources which are directly above the antenna, i.e. on the antenna axis, or at locations a few degrees above a plane perpendicular to the axis and passing through the antenna, or from sources located anywhere in the solid angle between these extremes. The radiation pattern is also characterised by an axial null in the direction opposite to the direction of maximum gain.
The bandwidth of the above described quadrifilar resonance is relatively narrow and, particularly in the case of miniature quadrifilar antennas having a core of a high dielectric constant, presents a manufacturing difficulty in achieving sufficiently close dimensional tolerances to be able repeatedly to produce antennas having the required cardioid response and resonant frequency.